1. Technical Field
The present invention relates to a semiconductor device and a method for manufacturing the same, more particularly, which is capable of reducing a self-heat generation effect and suppress a substrate floating effect.
2. Related Art
A field effect transistor which is formed on a silicon-on-insulator (SOI) substrate (hereinafter to be called an ‘SOI transistor’) is brought to attention for its utilities, in terms of easy device isolation, latch-up free, low source/drain junction capacitance, and the like. In particular, as a fully depleted SOI transistor is capable of operating at high speed with low power consumption and is easily driven by low voltage, research for operating the SOI transistor in a fully depleted mode has become increasingly popular.
Normally, an insulator of the SOI corresponds to silicon oxide (SiO2). A dielectric constant of silicon oxide is 3.9, which is small enough compared to the dielectric constant of silicon which is 11.9. However, an ultimate dielectric constant material is air. Therefore, a transistor which has air as an insulating layer, more particularly, a silicon-on-nothing (SON) transistor has started to gain attention.
A method for manufacturing an SON structure, for example, is disclosed in JP-A-2003-332540. More specifically, the JP-A-2003-332540 discloses a method for manufacturing a semiconductor substrate including a first step of implanting ions to form a microcavity in a desired region of a substrate, a second step of heat-treating the substrate that the microcavity was formed by the first step, and the second step of having at least a step of high temperature heat treatment which exposes the substrate to temperature exceeding 1000 degrees Celsius. In such a manufacturing method, by growing and uniting the microcavities formed on the substrate, an SON semiconductor substrate having a flat cavity roughly-paralleled to a substrate surface can be formed.
However, compared to silicon oxide and silicon, air has bad thermal conductivity. Therefore, operating temperature of an SON device (in other words, a device having the SON transistor) becomes higher compared to a conventional silicon device and an SOI device. A phenomenon which is called the self-heat generation effect deteriorates performance and reliability of the device (problem 1).
Further, in the SOI device, there is also a problem of the substrate floating effect which is caused by a body potential of the SOI transistor being in a floating state by complete device isolation. Consequences of the effect include, for example in an Nch transistor, a generation of a hole at the end of a drain by impact ionization, and because the hole is to be accumulated in the body, thereby results in a degradation of a breakdown voltage and the like (problem 2).